Fix for underscore templates being used in the yaml front matter.

This fix will process the underscore templates in the yaml front matter for use with in the page's data context. Previously, the page's data context was being skipped.

Also updated to use the data from the `context[basename]` instead of `context[filename]` since the `filename` property is the full name with an extension and the original goal was to be able to access the page content based on the files's basename... `alerts.success`.

The basename update is causing some weirdness when there is already a property on the context that isn't really the data from the file. You can see this when rendering the `assets` file since `assets` is a property on the assemble options object. The result is that the `page` property contains the value of `assets` which is wrong. This will need to be addressed in another issue.